Postdocs are the single most productive members in a research group. The PI is typically a tenured or tenure track professor, who got the job based on their re

Postdocs are the single most productive members in a research group. The PI is typically a tenured or tenure track professor, who got the job based on their research ability, but now they are relegated to an office, teaching 1-2 classes/semester, writing grants, reviewing papers for free for for-profit journals, reviewing grants for NIH/NSF, etc. Grad students are not well motivated, not skilled, need lots of training, take classes, need to work on a thesis, and often times also teach. So they are low output and high time investment. Postdocs require little-to-no time investment as they already have most prerequisite skills, have no other obligations beyond research, and sometimes even bring their own funding. They are basically professional researchers and are by far and away the most productive members of any given research group.